To unlock the hidden vinyl decals you must beat the following opponents and own their car:
Baron's vinyl - Beat Baron and own car
Bull's vinyl - Beat Bull and own car
Earl's vinyl - Beat Earl and own car
Izzy's vinyl - Beat Izzy and own car
Jade's vinyl - Beat Jade and own car
JV's vinyl - Beat JV and own car
Kaze's vinyl - Beat Kaze and own car
Ming's vinyl - Beat Ming and own car
Ronnie's vinyl - Beat Ronnie and own car
Sonny's vinyl - Beat Sonny and own car
Taz's vinyl - Beat Taz and own car
Vic's vinyl - Beat Vic and own car
Webster's vinyl - Beat Webster and own car
Complete the bonus part challenges to receive these parts:
SC Hood & Spoiler for Carrera 4S - Beat BPC 1&2 on Rank 5
SC Hood & Spoiler for Carrera GT - Beat BPC 1&2 on Rank 2
SC Hood & Spoiler for Cobalt SS - Beat BPC 1&2 on Rank 13
SC Hood & Spoiler for Corvette - Beat BPC 1&2 on Rank 6
SC Hood & Spoiler for Eclipse - Beat BPC 1&2 on Rank 11
SC Hood & Spoiler for Ford GT - Beat BPC 1&2 on Rank 3
SC Hood & Spoiler for Gallardo - Beat BPC 1&2 on Rank 4
SC Hood & Spoiler for Golf - Beat BPC 1&2 on Rank 15
SC Hood & Spoiler for Lancer - Beat BPC 1&2 on Rank 9
SC Hood & Spoiler for Mazda3 - Beat BPC 1&2 on Rank 14
SC Hood & Spoiler for Mustang GT - Beat BPC 1&2 on Rank 10
SC Hood & Spoiler for RX-8 - Beat BPC 1&2 on Rank 12
SC Hood & Spoiler for TT - Beat BPC 1&2 on Rank 7
SC Hood & Spoiler for WRX STI - Beat BPC 1&2 on Rank 8
Beat the following bosses to unlock these cars:
Carrera 4S - Beat Ming
Carrera GT - Beat Toru Sato
Corvette - Beat Kamikaze
Eclipse - Beat Big Lou
Ford GT - Beat JV
Gallardo - Beat Webster
Lancer & WRX STI - Beat Earl
M3 GTR 1 - Beat Ronnie
M3 GTR 2 - Beat Razor
Mustang GT - Beat Baron
RX-8 - Beat Izzy
TT - Beat Jewels